This invention relates to apparatus for removing fuel pellets from nuclear fuel elements and particularly to such apparatus utilizing internal pressurization of the fuel element.
In many designs of nuclear reactors, the reactor vessel has an inlet and an outlet for circulation of a coolant in heat transfer relationship with a core contained therein that produces heat. The core comprises an array or arrays of fuel assemblies which contain fuel elements. The fuel element is generally a cylindrical metallic sheath sealed at both ends containing nuclear fuel. The nuclear fuel which may be, for example, ceramic fuel pellets of a uranium compound, is stacked within the metallic sheath. During reactor operation, the nuclear fuel pellets fission releasing fission products such as fission gas while generating heat in a manner well known in the art.
During the manufacture of such fuel element it is necessary to stack the fuel pellets in the metallic sheath and seal the end thereof. Occasionally during this process a defect in the sheath, the stacking process or the fuel pellets may be discovered which would necessitate the scrapping of the particular fuel element. When a defective fuel element is so scrapped, it is a common practice to attempt to salvage as much of the fuel element as is possible. Since the fuel pellets are tightly contained within the metallic sheath, the methods of salvaging the fuel element have historically employed destructive means to remove the fuel pellets. For example, it is known in the prior art to salvage part of the fuel pellets by sectioning the metallic sheath and crushing the fuel pellets. It is also known to use lasers or machine tools to cut the metallic sheath open lengthwise and remove the fuel pellets. However, these techniques cause some of the fuel pellets and the metallic sheath to be damaged or contaminated with foreign material which resulted in a low percentage of recovery of nuclear fuel and the total destruction of the metallic sheaths.
In G.B. Pat. No. 1,097,597 to Guneratne et al there is described a method to remove metallic sheaths from nuclear fuel elements by applying a high frequency alternating electric current to the sheath so that the temperature of the sheath is raised relative to that of the fuel therein. Raising the temperature of the sheath causes the sheath to soften and expand so that the sheath may be removed by ultrasonic means or flexibly mounted scrapers or shredders. While this method may increase the salvage fuel pellet percentage, the scrapers or shredders will render the sheath unusable. Furthermore, Guneratne suggests that the alternating current be applied while the shredders are engaged, which would increase the complexity of the apparatus involved.
In addition to these concepts, fluid pressure has been employed to mold metal casings into a particular shape, but not to remove fuel pellets from a fuel element.